1. Field of the Invention
This invention relates to a polyester that is suitable for use as a material in the manufacture of articles such as a variety of parts for industrial use, film, sheets, and the like, and as a material for fibers, paints, adhesives, and the like. This invention also relates to articles that are made of this polyester.
More particularly, this invention relates to a polyester that can be used as a thermoplastic elastomer or as a liquid-crystal polymer for a variety of articles.
2. Description of the Prior Art
In general, if articles made of a composition that contains polymers are to have rubber-like elasticity, it is necessary for the polymer to be cross-linked at some locations in its molecule. For example, in rubber with elasticity, chains of rubber molecules are chemically cross-linked with sulfur molecules, thereby forming a network structure. Also, a number of compositions have been proposed that are combinations of a variety of thermoplastic polymers with a cross-linking agent. The composition undergoes a cross-linking reaction while the article is being formed from the composition. After the cross-linking reaction, the cross-linked composition is not thermoplastic any more, so it is not possible to mold the cross-linked composition by injection molding or by extrusion molding.
In recent years, thermoplastic elastomers have been developed that have rubber-like elasticity at ordinary temperatures and that are also plastic at high temperatures, and a variety of such thermoplastic elastomers are commercially available. For these thermoplastic elastomers, a long cross-linking process like that needed for rubber is unnecessary, and it is possible to form articles by injection molding or extrusion molding. The thermoplastic elastomers consist of soft segments and hard segments. The chemical structures of the soft segments and hard segments are different. In a mixture that contains both, regions that have similar properties collect together, and regions that have different properties mutually repel each other, resulting in an nonhomogeneous microscopic structure. The collected regions of the hard segments mentioned above act as physical cross-linkings between the polymer molecules. The physical cross-linkings arise only at ordinary temperatures or less, and thus, they are different from chemical cross-linkings.
The thermoplastic elastomers include, for example, polystyrene elastomers, polyolefin elastomers, polyurethane elastomers, polyester elastomers, polyamide elastomers, and other elastomers. In polystyrene elastomers, polystyrene segments correspond to hard segments, and form cross-linking domains. In other words, because the polystyrene segments bind polymer chains together, the polystyrene elastomers have elasticity. The hard segments of polyolefin elastomers are the crystalline phase of the polypropylene segments. In polyurethane elastomers, the polymer chains are cross-linked physically by hydrogen bonding of the polyurethane segments. In polyester elastomers, the polybutylene terephthalate segments act as the hard segments, and in amide elastomers, the nylon segments such as segments of nylon 6 or nylon 6,6 act as the hard segments.
As mentioned above, thermoplastic elastomers have rubber-like elasticity at ordinary temperatures, and are readily molded because chemical cross-linkings are not present. Thus, these materials are used in automotive parts and in a wide variety of other industrial products. However, because the cross-linking in conventional thermoplastic elastomers is achieved via physical binding, unlike the cross-linking in rubber, the heat stability is poor, as are the creep properties. Of all thermoplastic elastomers, the elastomer with the greatest heat stability is PELPREN.RTM. S-9001 available from Toyobo Co., Ltd. However, the melting point of this elastomer is at most 223.degree. C., and the heat deflection temperature (HDT) is 146.degree. C. The HDT is measured by method B of JIS K7207 (JIS K7207 corresponds to ASTM D648) for the deflection temperature under a load. The Vicat softening point A of polyurethane elastomers is at most 140.degree. C.
To raise the heat stability of conventional thermoplastic elastomers, it is necessary to increase the proportion of hard segments. However, if this is done, the hardness of the product at room temperature or lower temperatures is inevitably increased. Therefore, such elastomers are not suitable for products that should have elasticity over a wide range of temperatures, such as tubes, hoses, belts, packing, electrical wires, sporting goods, automotive parts, and the like.
In general, aromatic polyesters are obtained by the poly-condensation of the diols, dicarboxylic acids, and hydroxycarboxylic acids such as are shown in Table 1.
TABLE 1 __________________________________________________________________________ Heat Elongation Monomer components deflection Tensile at Dicarboxylic Hydroxycarboxylic temperature strength rupture Diol acid acid (.degree.C.) (kg/fcm.sup.2) (%) __________________________________________________________________________ Polyethylene Ethylene Terephthalic -- terephthalate glycol acid Polybutylene Butanediol Terephthalic -- 58 540 360 terephthalate acid Polycarbonate Bisphenol A Phosgene -- 138 630 110 Polyarylate Bisphenol A Terephthalic -- 164 715 60 acid and isophthalic acid Liquid-crystal Ethylene Terephthalic p-Hydroxybenzoic 64 1200 5 polymer glycol acid acid Liquid-crystal -- -- p-Hydroxybenzoic 180 2100 3 polymer acid and 2-hydroxy- 6-naphthalenec carboxylic acid Liquid-crystal 4,4'-Dihydroxy- Terephthalic p-Hydroxybenzoic 293 1200 6 polymer biphenyl acid acid __________________________________________________________________________
The HDT of the aromatic polyesters numbered 1 to 4 in Table 1 is less than 180.degree. C., so that the polyesters cannot be used, for example, as materials for surface-mounting electronic parts, and the like, which require high heat stability.
Polyesters that contain hydroxycarboxylic acid in the backbone chain of the aromatic polyester, such as the compounds numbered 5 to 7 in Table 1, are known as liquid-crystal polymers. Because hydroxycarboxylic acid is incorporated into the molecule, these polymers have liquid-crystal properties (i.e. mesomorphism). For that reason, when these liquid-crystal polymers are melted, their flowability is greater than that of ordinary polymers. In this way, liquid-crystal polymers have excellent moldability, and comparatively good heat stability. A polycondensate made of parahydroxybenzoic acid and 2-hydroxy-6-naphthalanecarboxylic acid is commercially available as the liquid-crystal polymer Vectra.RTM. (Celanese Chemical Co.). This polycondensate has liquid-crystal properties based on hydroxycarboxylic acid, so it has greater melting flowability than ordinary polymers. For that reason, its moldability is good when being molded by injection molding or the like (Japanese Patent Publication No. 57-24407).
However, as shown in Table 1, the heat stability of polyesters 5 and 6 is not satisfactory. The HDT of polyester 7, 293.degree. C., shows that its heat stability is satisfactory, but when this polyester is to be molded by, for example, injection molding, it is necessary to keep the cylinder temperature at 360.degree. C. or more, so its moldability is poor.
As methods for the lowering of the melt viscosity of polyesters, the molecular weight of the compound can be decreased, or it might be possible to add plasticizer or a processing aid. With the first method, mechanical properties such as tensile strength and impact resistance decline, and the heat stability of the product is so much decreased as to make it unusable. With the second method, the disadvantage is that the product obtained has decreased heat stability.
In general, the following methods have been proposed for the raising of the heat stability of aromatic polyesters.
1. The blending of reinforcing fibers such as glass fibers or the like in the aromatic polyester.
2. The raising of the molecular weight of the polyester, or the increase of the cross-linking density of the polyester.
However, with method 1, the moldability is decreased, and the surface of the molded product is rough. If a composition that contains glass fibers in the above-mentioned aromatic polyester is used for the molding of products that are light, weighing about 10 mg to 10 g, that have thin parts, 1 mm or less thick, or that have sharp corners on the tip such as gears, the proportion of glass fibers in the thin parts or the sharp corners is less than that in the body of the product, because the fluiability of the glass fibers therein is poor. The result is that a satisfactory reinforcing effect arising from the glass fibers is not attained in the thin parts or the sharp corners. In addition, anisotropy and warping occur based on the orientation of the glass fibers, and it is not possible to achieve accurate molding.
With method 2, because the melt viscosity increases, the moldability is decreased.
A widely known polyester obtained from ethylene glycol and terephthalic acid is polyethylene terephthalate (PET). PET is a resin that has superior mechanical properties, electrical properties, flame-retarding properties, weatherability, resistance to chemicals, and the like.
However, PET has a slow rate of crystallization, so ordinarily, when it is injection-molded in a mold that is heated to 100.degree. C. with the use of boiling water, PET solidifies rapidly so that crystallization of the resin cannot proceed sufficiently in the mold. For that reason, the dimensional stability of the articles obtained is poor, and the condition of the surface of the articles is not satisfactory. When PET is formed in a mold that is heated at a temperature of 130.degree. C. or more, these disadvantages that arise from the slow rate of crystallization are overcome. However, in this case, the mold must be heated to a high temperature, which is economically disadvantageous.
In order to improve the crystallinity of PET, a variety of methods have been tried. For example, there is a method (1) in which the movement of polymer molecules is accelerated by the addition of a plasticizer, or by the copolymerization of a monomer, which confers plasticity on the polymer, by which means the molecular orientation needed for crystallization is promoted. Also, there is a method (2) in which crystallization can be promoted by the addition beforehand of an agent to accelerate crystallization.
Method 1 is disclosed in Japanese Patent Publication No. 47-3027, Japanese Patent Publication No. 47-4140, and Japanese Laid-Open Patent Publication No. 57-38849, in which it is disclosed that ethylene glycol or the like that forms soft segments is introduced into the polymer chain of an aromatic polyester. However, with this method, it is not possible to avoid declines in the heat stability, mechanical properties, and the like of the aromatic polyester.
Method 2 is disclosed in Japanese Laid-Open Patent Publication No. 54-158452, Japanese Laid-Open Patent Publication No. 56-57825, and Japanese Patent Publication No. 54-38622, in which the addition of a metal salt of an organic acid and PET with a high melting point is disclosed. However, when this kind of agent to accelerate crystallization is added, there are limits to the acceleration of the crystallization, and it may not be possible to accelerate the crystallization sufficiently.
Methods similar to the above include that disclosed in Japanese Laid-Open Patent Publication No. 55-82150, in which a specific crystallized compound is added to PET. However, the rate of crystallization of aromatic polyester with this method is slow. In Japanese Laid-Open Patent Publication No. 56-104933, a method is disclosed in which a compound that confers a liquid crystal segment structure is bound to the polyester molecules. However, with this method, the rate of crystallization of aromatic polyesters is not satisfactory.